If the toolchain is packaged for later use as external toolchain, the resulting
tarball is created in $BIN_DIR. But without building all packages first that
directory isn't created, hence 'make target/toolchain/compile' fails when
trying to create the toolchain tarball with error "Cannot open: No such file or
directory".

To fix that the $BIN_DIR is created before using it.

Signed-off-by: Micha Lenk <micha@lenk.info>